The present invention relates to a light-sensitive silver halide photographic material for forming direct-positive images, in particular computer output (COM) images, and also relates to a method for producing such a light-sensitive direct-positive silver halide photographic material.
With recent rapid progress of information transmitting systems, silver halide photographic materials have been increasingly required to have high sensitivity. This high sensitivity is desired as well in the field of negative working as of positive working silver halide photographic materials. A negative working photographic material is known as one giving a negative image while a positive working photographic material is known as one giving a positive image. The term xe2x80x9cnegative imagexe2x80x9d means that a reversal image appears while the term xe2x80x9cpositive imagexe2x80x9d indicates that no reversal image as an intermediate image appears but that a reproduction of the original image directly appears.
In silver halide photography a photographic method, according to which a positive image is made, is therefore called a direct-positive method and a photographic light-sensitive material and a photographic emulsion for use according to such direct-positive method are called direct-positive material and direct-positive emulsion respectively.
Because of their practical and economical usefulness in the field of e.g. printing out of computer information preference is given nowadays to the use of direct-positive materials and direct-positive emulsions for this purpose.
A variety of direct-positive photographic methods and materials are known. The most useful and well-known methods are following. One method starts subjecting a photographic material comprising silver halide grains that have light-sensitive specks mainly inside the grains to an image-wise exposure and developing the exposed material in the presence of a development nucleator or developing the exposed material after overall light-flashing it to fog. The other method starts exposing a photographic material comprising prefogged silver halide grains to light in the presence of a desensitizing agent and ends with classical processing steps. Such type of emulsion is commonly known as Herschel reversal emulsions and has e.g. been described in U.S. Pat. No. 3,367,778. Silver bromide or silver bromoiodide grains are normally preferred. Said first type can however not be developed in a classical surface developer and requires a supplementary fogging treatment subsequent to the image-wise exposure before or while applying a normal surface development.
The present invention relates to the method wherein use is made of prefogged silver halide emulsions.
Nowadays COM-systems on the market normally make use e.g. of a He/Ne laser (633 nm) and for very short exposure times (flash exposure times of from 10xe2x88x924 up to even 10xe2x88x926 seconds) it remains difficult to attain the desired sensitivity level. Materials sensitized for light with a wavelength between 600 nm and 700 nm are moreover, as is generally known, not so high in sensitivity. Various proposals have been made in an attempt to solve this problem. So it is a primary condition to spectrally sensitize the emulsion crystals. An optimized spectral sensitization leading to an aborption maximum closely matching the desired maximum absorption wavelength (e.g. at 633 nm in case of Hexe2x80x94Ne laser exposure) is thus highly requested.
It has moreover been found that it is favourable to enrich the surface of prefogged grains with iodide ions, as has e.g. been disclosed for the silver chlorobromoiodide grains described in U.S. Pat. No. 5,501,939; in order to provide iodide hole trapping centers, thereby improving the efficiency of the bleaching process of fog centers.
Opposite to the expected increase in sensitivity it has in praxis been established however that such enrichment of the grain surface with silver iodide may even lead to loss in sensitivity, due to a shift in absorption maximum of the spectral sensitizer, in that the said maximum now appeared at a more bathochromic wavelength instead of the wavelength of 633 nm as desired in case of Hexe2x80x94Ne laser exposure.
It is an object of the present invention to provide a silver halide photographic material comprising a layer containing a spectrally sensitized prefogged direct-positive silver halide emulsion having prefogged grains or crystals rich in silver bromide with a grain surface enriched in silver iodide providing an optimized high sensitivity in the range from 600-700 nm, representing the xe2x80x9cred lightxe2x80x9d wavelength range (e.g. 633 nm as wavelength in case of a Hexe2x80x94Ne laser exposure), wherein said material, developable in a classical surface developer without a supplementary treatment, provides images of good quality.
It is another object of the present invention to provide a method for obtaining said photographic material.
It is further object of the present invention to provide a method for obtaining an image with said photographic material.
It is a still further object of the present invention to provide a method for obtaining with said photographic material an imaging element having a high sensitivity in COM-applications.
Further objects of the present invention will become apparent from the description hereinafter.
The above mentioned objects are realized by providing a black-and-white silver halide photographic material coated on a support with at least one light-sensitive emulsion layer, comprising a spectrally sensitized prefogged direct-positive silver halide emulsion, providing peak absorption in the wavelength range from 600 nm up to 700 nm, wherein said emulsion comprises a binder and core-shell emulsion crystals having silver bromide in a total amount of at least 80 mole %, further having, in a shell of said core-shell emulsion crystals, silver iodide in a range from 0.5 up to 20 mole %, based on silver, said shell representing less than 50 mole %, and, more preferably not more than 25 mole %, of all silver precipitated, characterized in that said emulsion is spectrally sensitized with a combination of a desensitizing dye having an absorption maximum wavelength in a range from 600 nm up to 700 nm, if present as a sole dye in said emulsion, and at least one azacyanine dye having an absorption maximum at a more hypsochromic wavelength. In a preferred embodiment said maximum wavelength and said hypsochromic wavelength are differing at least 50 nm, and, more preferably, more than 100 nm. It is thus clear that the desensitizing dye, if present as a sole dye, has an absorption maximum at a more bathochromic wavelength versus the said peak absorption and that presence of said at least one dye or a combination of dyes shifts the absorption spectra towards the desired peak absorption wavelength.